The present invention concerns a fuel cell electrode and matrix, and specifically concerns an integral structure comprising a sheet-form electrode and matrix, and a method for manufacturing the same.
In the case of fuel cell electrodes, the fuel cell reaction proceeds as a result of the formation of interfaces between three phases, i.e., reaction gas, electrolyte and catalyst. Accordingly, an interface between the gas and the electrolyte must be formed inside the electrode. Conventionally, an electrode consisting of (a) a powdered conductive substance containing a catalytic substance, and (b) a polytetrafluoroethylene (PTFE) resin has been used for this purpose. Such electrodes have generally been formed as follows: a mixture of finely powdered PTFE and a powdered conductive substance containing a catalytic substance is uniformly mixed with an appropriate solvent, and is then painted or sprayed onto a substrate consisting of a porous carbon material (e.g., porous carbon paper) which has been subjected to a water-repelling treatment. Alternatively, uniform molding may be achieved by screen printing or the mixture may be further kneaded following mixing, spread to form a sheet, and integrally molded by pressing onto a substrate of the aforementioned type. Electrodes obtained by these manufacturing methods have almost no strength in themselves. Accordingly, a porous carbon substrate has been essential, and an electrode could only be formed on the basis of the strength of this substrate.
Characteristics such as those described in (1) through (5) below are required in a fuel cell matrix.
(1) The matrix must be stable against hot concentrated phosphoric acid under the operating conditions of the fuel cell.
(2) The matrix must be an electronic insulating material, and must also have sufficiently high ionic conductivity.
(3) The matrix must have the ability to block the permeation of gas when impregnated with phosphoric acid; i.e., the matrix must have a high foam pressure.
(4) The matrix must have a high affinity for phosphoric acid, so that the osmotic diffusion of phosphoric acid is rapid.
(5) The matrix must possess mechanical strength and be formable into a thin layer.
Matrix materials using a phosphoric-acid-resistant inorganic powder such as SiC or Ta.sub.2 O.sub.5, and a small amount of PTFE as a binder are conventionally well known as matrix materials which provide such required characteristics. Generally, such matrix materials are formed as follows: an appropriate amount of an appropriate solvent is added to and mixed with the aforementioned inorganic powder and a minute amount of PTFE; this mixture is sprayed or painted on the electrode so that it adheres in the form of a thin film; and, finally the solvent is removed. Alternatively, a method may be employed in which the mixture is further kneaded and spread to form a sheet, after which said sheet is pressure-bonded to the electrode. Furthermore, matrix materials are also known in which a fine carbonaceous powder such as carbon black is used in order to raise the foam pressure described in (3) above, and in order to increase the osmotic diffusion described in (4) above. In this case, a PTFE binder is used as described above, and a sheet formed in the same manner is combined with a matrix using the aforementioned fine inorganic powder so that a two-layer or three-layer matrix is obtained.
However, in all of the abovementioned conventional matrix materials, PTFE is used as a binder. Since this PTFE has strong water-repelling characteristics, large amounts of binder PTFE cannot be used in order to achieve the aforementioned affinity for phosphoric acid described in (4) above. Accordingly, the mechanical strength described in (5) above is extremely low. As a result, a matrix function can be achieved only by forming the matrix on a rigid electrode substrate or electode. Accordingly, both reliability and yield are unavoidably low.